1. Field of the Invention
The present invention relates to a molding evaluating device for evaluating the molding stability or the like of molded pieces produced by an injection molding machine and a method for evaluating the same.
2. Statement of the Prior Art
In an injection molding machine for producing resin molded pieces, a screw is normally provided in a cylinder in such a manner as to move back and forth therein. The screw is advanced after the cylinder has been filled with a molten resin material so as to inject the molten resin material into a metal mold, and the resin material so injected into the metal mold is then cooled so as to be set up therein, a molded piece in a desired shape being thereby produced.
In order to provide molded pieces of a certain quality without fail, every time a molded piece is produced by the above injection molding machine, it is necessary to determine whether the molded piece so produced is good or defective. However, it is practically not possible for workers to accurately determine whether all molded pieces so produced are good or defective. In order to cope with this, there have been heretofore disclosed devices for automatically determining whether molded pieces are good or defective. For instance, the gazette of Japanese Patent Laid-Open No. 62-187009 discloses a device wherein waveform data showing a relationship between the position of a screw and the pressure (injection pressure) imparted to a molten resin material by the screw in a stage where good molded pieces are able to be produced in a stable manner are obtained and stored in a memory in advance, and wherein afterwards, every time a molded piece is produced waveform data showing a relationship between the position of the screw and the injection pressure when the molded piece is so produced are obtained for comparison with the waveform data already stored in the memory, the molded piece so produced being thereby determined as being good or defective.
However, with this method of determining whether molded pieces are good or defective, it is difficult to make a highly accurate determination, because in this method, molded pieces are determined as being good or defective in a qualitative fashion by comparing waveform data of two types with each other, in other words, recognizing waveform patterns.
In order to solve the problem inherent in the above prior art device, there have also been heretofore disclosed methods for determining whether molded pieces are good or defective in a quantitative manner. For example, the gazette of Japanese Patent Laid-Open No. 62-134239 discloses a method comprising the steps of detecting the injection speed and injection pressure of a screw, time integrating a value obtained by multiplying the injection speed by the injection pressure so detected during the time from the start of injection until the end of dwelling so as to obtain the total amount of energy (hereinafter referred to as "work done") imparted to a molten resin material by the injection molding machine via the screw, and then comparing the work done with a reference value set in advance so as to determine whether a molded piece is good or defective. Thus, in this example, since every time a molded piece is produced, a quantitative value (work done) in relation to the production is obtained, and since molded pieces are determined as being good or defective on the basis of the values so obtained, it is possible to take a determination more highly accurate than one in the case of the above-mentioned prior art device.
By the way, in a technical field of this kind, it is desirable not only to determine whether a molded piece is good or defective but also to monitor whether or not good molded pieces are continuously produced in a stable fashion (hereinafter referred to as "molding stability"), as well as how the temperature of a metal mold or the viscosity of a molten plastic material changes. However, there has been heretofore disclosed no device satisfying such a requirement.